NLog schema, XSD won't work for new FilteringWrapper syntax

related https://github.com/NLog/NLog/issues/3472

after fix, then:

works (no XSD errors):

    <target xsi:type="FilteringWrapper" name="default" >
      <filter xsi:type="whenRepeated" layout="${message}" timeoutSeconds="30" action="Ignore" />
      <filter xsi:type="when"/>
    </target>

works:

    <target xsi:type="FilteringWrapper" name="default" >
      <target xsi:type="Console"  />
    </target>

not working

    <target xsi:type="FilteringWrapper" name="default" >
      <filter xsi:type="whenRepeated" layout="${message}" timeoutSeconds="30" action="Ignore" />
      <filter xsi:type="when"/>

      <target xsi:type="Console"  />
    </target>

he element 'target' in namespace 'http://www.nlog-project.org/schemas/NLog.xsd' has invalid child element 'target' in namespace 'http://www.nlog-project.org/schemas/NLog.xsd'. List of possible elements expected: 'name, condition, filter, optimizeBufferReuse' in namespace 'http://www.nlog-project.org/schemas/NLog.xsd'.

The issue is that in the XSD, it's a "choice":

  <xs:complexType name="FilteringWrapper">
    <xs:complexContent>
      <xs:extension base="WrapperTargetBase">
        <xs:choice minOccurs="0" maxOccurs="unbounded">
          <xs:element name="name" minOccurs="0" maxOccurs="1" type="xs:string" />
          <xs:element name="condition" minOccurs="0" maxOccurs="1" type="Condition" />
          <xs:element name="filter" minOccurs="0" maxOccurs="1" type="Filter" />
          <xs:element name="optimizeBufferReuse" minOccurs="0" maxOccurs="1" type="xs:boolean" />
        </xs:choice>

Start with the XSD definition of the FilteringWrapper complexType shown in the issue and reproduce the three XML validation cases. Update the schema so a FilteringWrapper can contain both filter elements and a nested target, then verify that the previously failing configuration validates without breaking the two working examples.